The present invention is directed to an exhaust gas purification and, more particularly, to an exhaust gas purifying apparatus for an automotive vehicle engine of the type having a plurality of engine cylinders.
U.S. Pat. No. 3,908,371, which was patented on Sept. 30, 1975, discloses an exhaust gas purifying apparatus for an automotive vehicle, which uses a dual-catalyst exhaust treatment system which includes a reducing catalyst for removing an excess of oxygen and reducing nitrogen oxides in the exhaust gases and an oxidizing catalyst for oxidizing carbon monoxide and hydrocarbons in the exhaust gases. More specifically, according to the invention in the above described U.S. patent, a plurality of exhaust ducts which are respectively connected to exhaust outlets of corresponding engine cylinders merge into a single exhaust manifold having reducing and oxidizing catalyst units disposed therein so that the exhaust gases emitted from the engine cylinders subsequent to combustion of an air-fuel mixture within the engine cylinders can, after having been joined together, pass through the reducing catalyst unit and then through the oxidizing catalyst unit prior to the exhaust gases being discharged to the atmosphere. In order to enhance the oxidization of carbon monoxide and hydrocarbons in the exhaust gases which have passed through the reducing catalyst unit, the purifying apparatus of the above described U.S. patent further comprises a secondary air supply means including an engine operated pump for introducing a secondary air flow into a portion of the exhaust manifold between the reducing catalyst unit and the oxidizing catalyst unit.
In the prior art dual-catalyst purifying apparatus having a construction as described above, the engine operated pump for introducing the secondary air into the manifold portion between the reducing and oxidizing catalyst units is required. The reason for this is that all of the exhaust ducts, which are respectively connected to the individual engine cylinders, are bundled into the single exhaust manifold at a position upstream of the reducing catalyst unit in terms of the direction of flow of the exhaust gases towards the atmosphere. More specifically, as is well known to those skilled in the art, automobile exhaust gases which are successively emitted from the engine cylinders generally flow through an exhaust manifold towards the atmosphere in a substantially pulsating manner with the pressure inside the exhaust manifold alternately increasing and decreasing because of the different timing of firing of air-fuel mixtures within the respective engine cylinders.
While it is well known to those skilled in the art that if all of the exhaust gases successively emitted from the individual engine cylinders are allowed to pass through the reducing catalyst unit such as in the prior art dual-catalyst purifying apparatus, the pulsating characteristic of flow of the exhaust gases through the exhaust manifold downstream of the reducing catalyst unit is weakened so that the prior art dual-catalyst purifying apparatus cannot make use of the pulsating characteristic to supply the secondary air into the exhaust manifold portion downstream of the reducing catalyst unit and upstream of the oxidizing catalyst unit, and an external pumping means, such as the engine operated pump, is accordingly required.
The use of the engine operated pump results in a substantial increase in the cost of manufacture of the purifying apparatus which in turn affects the cost of manufacture of an automotive vehicle equipped with such purifying apparatus. In addition, since the engine operated pump is driven by the power output shaft of the engine, the power output shaft has an unnecessary load added which substantially reduces the power output of the engine as a whole.
The copending U.S. patent application Ser. No. 891,111, which was filed on Mar. 28, 1978, in which two of the inventors therein are the same inventors as some of the present invention and the noted application assigned to the same assignee of the present invention, discloses a dual-catalyst purifying apparatus which substantially eliminates the disadvantages and inconveniences inherent in the above described prior art purifying apparatus. According to the invention in the copending U.S. patent application, a plurality of exhaust ducts are respectively connected to cylinders of the engine to allow an outward flow and the cylinders are divided into first and second groups, the exhaust ducts of the first group are connected to a main exhaust manifold having reducing and oxidizing catalyst units disposed therein while the exhaust ducts of the second group are connected to a portion of the main exhaust manifold between the reducing and oxidizing catalyst units through an auxiliary exhaust manifold.
For supplying a secondary air necessary to enhance oxidization of the exhaust gases during the passage of the exhaust gases through the oxidizing catalyst unit, there is a portion of the auxiliary exhaust manifold connected to a source of secondary air through a pressure operated valve which is in the form of a reed valve.
The dual-catalyst purifying apparatus disclosed in the copending U.S. patent application is designed so that, since there is no substantial resistance to flow of the exhaust gases in the auxiliary exhaust manifold, the exhaust gases which are successively emitted from the exhaust ducts of the second group, at intervals which are determined by the timing of successive firing of the air-fuel mixture within the respective engine cylinders, flow through the auxiliary exhaust manifold in a pulsating manner with the pressure alternately increasing and decreasing and this pulsating flow of the exhaust gases through the auxiliary exhaust manifold being used to repeatedly open and close the reed valve.
The dual-catalyst purifying apparatus which is disclosed in the copending U.S. patent application is satisfactory for removing or reducing the monoxides and hydrocarbons contained in the exhaust gases to be discharged to the atmosphere, and reduces them in an appreciably great amount when compared to the prior art dual-catalyst purifying apparatus. However, in the dual-catalyst purifying apparatus disclosed in the copending U.S. patent application, since the exhaust gases emitted from the exhaust ducts of the second group do not flow through the reducing catalyst unit, but only through the oxidizing catalyst unit, the employment of an extra means for reducing or removing the nitrogen oxides is required.